Underwater oil storage

ABSTRACT

An underwater oil storage apparatus having an oil storage tank moored to the sea bottom and a buoyancy adjusting tank secured to the oil storage tank, characterized by producing a substantially constant small buoyancy for a whole oil storage equipment by introducing or draining water in the buoyancy adjusting tank according to the amount of stored oil.

United States Patent 1 Toyama Sept. 24, 1974 UNDERWATER OIL STORAGE [75]Inventor:

[73] Assignee: Mitsui Shipbuidling and Engineering Co., Ltd., Tokyo,Japan 22 Filed: Sept. 8, 1972 21 Appl. No.: 287,222

Yoshio Toyama, Kanagawa, Japan [52] U.S. Cl 114/.5 T

[51] Int. Cl B63b 35/00 [58] Field of Search 114/.5 T

[56] References Cited UNITED STATES PATENTS 3,408,971 11/1968 Mott114/.5 T 3,710,582 1/1973 Hills et a1. 114/.5 T

3,719,048 3/1973 Arne et a1. 114/.5 T

Primary Examiner-Trygve M. Blix Assistant Examiner-Stuart M. GoldsteinAttorney, Agent, or Firm-Dorfman, Herrell and Skillman [5 7] ABSTRACT Anunderwater oil storage apparatus having an oil storage tank moored tothe sea bottom and a buoyancy adjusting tank secured to the oil storagetank, characterized by producing a substantially constant small buoyancyfor a whole oil storage equipment by introducing or draining water inthe buoyancy adjusting tank according to the amount of stored oil.

1 Claim, 4 Drawing Figures UNDERWATER OIL STORAGE The present inventionrelates to underwater oil storage.

The present invention has for its object to obtain an oil storageapparatus capable of storing oils in the deep sea.

The sea-bottom oil storage tank according to the present invention is soarranged that an oil storage tank made of metallic plate orferroconcrete is provided at a considerable depth under the sea and ismoored to the sea bottom by mooring lines and the buoyancy of the tankis controlled by the buoyance adjusting tanks mounted outside the tankso that excessive upward tension is not exerted on the mooring lines andthe sinker weights connected to the lower ends of the mooring lines. Thestorage tank is filled with water before storing oil, and the sea wateris substituted by oil in storing. Since the specific gravity of oil isfairly smaller than that of sea water, the tank has a buoyancy largerthan the structure weight when the tank is full of oil. However, in thecase of the tank full of sea water, it will sink to the sea bottomunless a buoyancy tank having a buoyancy larger than the structureweight is mounted on the storage tank. However, if the storage tank withthe buoyancy tank is filled with oil, the storage tank is subjected to abuoyancy equal to difference between the specific gravities of oil andsea water, and the force exerted on the sinker weights and mooring linesamounts to about 20 percent of the capacity of tank. Hence, the sinkerweights and mooring lines must be large beyond the limit ofpracticability.

In accordance with the present invention, water is introduced into thebuoyancy tank as much as to cancel the buoyancy increase caused by oilintroduced into the storage tank and, reversely, the sea water in thebuoyancy tank is drained by compressed air or a pump when the oil in thestorage tank is removed, thereby the buoyancy of an oil storageequipment as a whole is maintained at an approximately constant smallvalue.

In drawings:

FIG. 1 is a partly cross-sectional front view of an embodiment of anunderwater oil storage tank controlled according to the presentinvention;

FIG. 2 is a plan view thereof;

FIG. 3 is a schematic view explaining the relation between a storagetank storage quantity and the water supply quantity to a buoyancyadjusting tank; and

FIG. 4 is a diagram showing the relation between buoyancy and watersupply quantity.

A storage tank 1 is a spherical tank made of metallic plate as steelplate or made of ferroconcrete, and at the lower end thereof is provideda port 4 so as to balance the exterior and the interior pressures.

The storage tank is moored at three points by mooring lines 5 to sinkerweights 9 on the sea bed. A circular buoyancy adjusting tank 2 issecured to the storage tank at the periphery thereof.

At the top of the storage tank is mounted an oil supply or dischargingports 3, and oil supply or discharge pipes 6 run from the oil supply anddischarge ports 3 along the meridian of the storage tank to its bottomportion and are connected to flexible pipes 7. The flexible pipes 7 arecollected inside the central sinker weight 10 on the sea bottom andcommunicated to a sea-bottom pipe 8. At the top of the buoyancyadjusting tank is provided a port 11 for introducing and dischargingair, and at the bottom is provided a water inlet-andoutlet port 12. Fromthe port 11, an air pipe 13 is led to the bottom portion of the storagetank along the meridian of the storage tank and connected to a flexibletube 14, the lower end of which is fixed to the sinker weight 10, andfurther communicated to a sea-bottom air pipe 15. The sea-bottom oilpipe 8 and air pipe are connected to a sea berth or one-point buoy, andthe oil supply pipe is connected to a tanker and the air pipe is open tothe atmosphere in the case of introducing or discharging water by thepump, or connected to the compressed air source. In case the storagebody is composed of steel plates, its own weight is small. Accordingly,a fixed ballast vessel 16 is provided and ballast 17 is containedtherein so that the total weight of the equipment in the case the tankis full of oil may be approximately equal to the buoyancy at that timeas hereinafter described.

The operation of this equipment will now be described. In the conditionthat the present equipment is built up underwater, the storage tank 1 isfilled with sea water and the buoyancy adjusting tank is empty andfilled with air equal in pressure to the outside sea pressure oroccupied by air communicating the atmosphere by closing the sea waterinlet 12, where the buoyancy acting on the whole equipment is slightlylarger than the weight of the equipment excluding the weight of thesinker weight. The sinker weight 9 is determined to have weight enoughto hold that buoyancy, and therefore a small amount of tension acts onthe mooring lines 5 so that the equipment is floated in the sea. Whenoil is introduced into the storage tank, for example by operating a pump8a, buoyancy increases due to the difference of specific gravity fromwater. In order to cancel the increase of buoyancy, water is introducedinto the buoyancy adjusting tank 2 in the predetermined amount byquantity of oil to be introduced and the specific gravity, so that thebuoyancy is controlled not to deviate from the initially adjusted value.This control is performed by exhausting the compressed air from the tank2, for example by operating a pump 15a, or opening the valve forintroducing water. Reversely, in case of discharging oil, buoyancy isadjusted by discharging water in the buoyancy adjusting tank.Discharging of water is performed by supplying compressed air or by anunder water pump mounted on the under face of the buoyancy adjustingtank. A suitable controller for performing these controls is showndiagrammatically at 20 in FIG. 1.

The relationship between amounts of oil in the stor age tank and of thewater supplied to the buoyancy adjusting tank will now be described.

In FIGS. 3 and 4, the designation is made as follows:

Vm Volume of the storage tank rn' Vr Volume of the buoyancy adjustingtank m V0 Amount of storage oil in Vw Amount of water in the buoyancyadjusting tank m W Weight of structure (in water) ton p0 Specificgravity of oil ton/m pw Specific gravity of water ton/m T0 Tensionacting on mooring line ton The minimum necessary capacity of a buoyancyad'- justing tank is determined by the following formula.

where To is a value so determined that the equipment may not be inclinedby tidal current or other factors.

Buoyancy B increasing in the storage tank when oil is loaded isrepresented by the formula (2).

acting on the mooring line equal to To, is given by the followingequation.

This relationship is shown in graph as in FIG. 4. The minimum specificgravity of oil loadable in case Vm and Vr are determined is given by thefollowing:

Vm (pw-p) VrpwT0=0 and rewritten as follows pw pw (1 Vr/Vm) To/Vm SinceTo/Vm is negligibly small,

p0 z pw (1 Vr/Vm) When Vr 0.2 Vm and the outside is sea water of 4 1.025in specific gravity,

pu= L025 X 0.8 x 0.82

Accordingly, all kinds of crude oil may be fully loaded.

In accordance with the present invention, an oil storage tank of greatvolume may be moored to the sea bottom of to meters depth as notobstructing the navigation of ships and not affected by waves understormy weather, and the size of mooring line and the weight of sinkerweight is prevented from growing too large by buoyancy adjusting device.It will be understood that the sinker weights may be substituted byanchor bolts.

What is claimed is:

l. A method of controlling the buoyancy of an underwater oil storageapparatus comprising an oil storage tank, a buoyancy adjusting tanksecured to the oil storage tank, means for mooring the oil storage tankto the sea bottom, a port provided in the oil storage tank for oil, aport provided in the oil storage tank for water, a port provided in thebuoyancy adjusting tank for water, and a port provided in the buoyancyadjusting tank for air, comprising the steps of charging and dischargingoil and water into or from the oil storage tank in displacing relation,charging and discharging air and water into or from the buoyancyadjusting tank in displacing relation, and controlling said charging anddischarging of the storage tank and the discharging and charging ofthebuoyancy tank together to charge water into said oil storage tankconcurrently with discharge of water from said buoyancy adjusting tankand vice versa, whereby the floating oil storage apparatus is controlledto have a relatively small buoyancy at all times.

1. A method of controlling the buoyancy of an underwater oil storageapparatus comprising an oil storage tank, a buoyancy adjusting tanksecured to the oil storage tank, means for mooring the oil storage tankto the sea bottom, a port provided in the oil storage tank for oil, aport provided in the oil storage tank for water, a port provided in thebuoyancy adjusting tank for water, and a port provided in the buoyancyadjusting tank for air, comprising the steps of charging and dischargingoil and water into or from the oil storage tank in displacing relation,charging and discharging air and water into or from the buoyancyadjusting tank in displacing relation, and controlling said charging anddischarging of the storage tank and the discharging and charging ofthebuoyancy tank together to charge water into said oil storage tankconcurrently with discharge of water from said buoyancy adjusting tankand vice versa, whereby the floating oil storage apparatus is controlledto have a relatively small buoyancy at all times.